Brillouin-zone-folded intrinsic chirality tailoring third-harmonic generation in planar metasurface-waveguide systems

Robust chiral resonances with ultrahigh quality factors (Q-factors) and strong circular dichroism (CD) underpin both fundamental studies of optical chirality and emerging chiral photonic technologies. Here, we theoretically propose a paradigmatic strategy for realizing such resonances in planar metasurface-waveguide systems by harnessing Brillouin-zone folding. Using an elliptical-disk dimer metasurface as a representative platform, we show that Brillouin-zone folding combined with in-plane C2 symmetry breaking naturally yields two distinct polarization-selective resonances, namely an x-polarized quasi-guided mode and a y-polarized quasi-bound state in the continuum. When the remaining in-plane mirror symmetry is broken, these orthogonally polarized resonances become simultaneously accessible and coherently hybridize, producing a pronounced chiral mode with a Q-factor approaching 103 and CD of 0.997. Moreover, nonlinear simulations further reveal that this intrinsically chiral resonance provides a powerful handle for tailoring optical nonlinearities, boosting third-harmonic generation to a conversion efficiency of 3.8 × 10−6 under a pump intensity of 1 MW cm−2. Notably, the third-harmonic response is highly spin-selective, exhibiting nearly three orders of magnitude contrast between left- and right-circularly polarized excitation. Our results establish Brillouin-zone folding combined with symmetry engineering as a versatile route to high Q-factor chiral resonances, offering opportunities for chiral photonic applications across both linear and nonlinear domains.